The present invention is directed to electromagnetic interference (EMI) shielding and is more particularly directed to new materials, e.g., gaskets, caulking compounds, adhesives, coatings, etc., useful for EMI shielding.
Today EMI shielding takes many forms depending upon the application. Where high shielding performance is necessary, EMI shielding has tended to use silver particles or silver coated copper particles dispersed in a resin binder. The EMI shielding product has been formed into gaskets of various shapes, sheets or provided as caulking compounds, adhesives, coatings, etc.
While it is still preferred from a technical standpoint that pure silver particles based EMI shielding materials be used, the cost of silver has made this economically impracticable for most applications.
The use of silver coated copper which has usually been a substitute for silver particles has certain disadvantages, particularly with respect to ageing characteristics. The maximum continuous temperature for silicone rubber gaskets filled with silver coated copper particles is generally considered to be about 125.degree. C. At higher temperatures, oxidation of the exposed copper tends to result in loss of electrical conductivity. In addition, on exposure to salt spray, corrosion products formed on exposed copper are green colored. In general, salt spray corrosion does not severely affect either electrical or physical properties, but the green corrosion products are unsightly.
The art is also replete with the use of other types of electrically conductive particles in EMI shielding materials. One of those in substantial use at this time is silver coated glass.
EMI shielding gaskets employing current state of the art silver coated glass particles have under certain vibration conditions experienced increased resistance and thus became substantially ineffective as a shield.
In addition, silver coated glass particles because they are not an all metallic material do not have high current carrying capacity which is necessary if an electromagnetic pulse condition is encountered.
Reference may be had to U.S. Pat. Nos. 3,140,342, 3,194,860, 3,202,488, 3,476,530 and 3,583,930 for a description of the state of the art.
The present invention provides an EMI shield which avoids some of the cost and other disadvantages of silver coated particle EMI compositions (materials) of the prior art while at the same time providing almost the same EMI shielding effectiveness as EMI shielding compositions using solid silver particles. Unexpectedly it has been discovered in this invention that it is possible to use silver coated particles using about one third less silver than used on silver coated copper particles and yet achieve about as good conductivity as that obtained when using silver coated copper particles as the electrically conductive particle. In addition, the maximun continuous use temperature has been found to be about 200.degree. C. using a silicone binder.
The present invention also has the advantages in comparison with silver coated copper in that the characteristic green corrosion is no longer a factor.
The weight of the EMI shield has also been significantly reduced in this invention in comparison to silver or silver coated copper based EMI shielding systems, a significant factor for aerospace applications.